Blackrazor, Devourer of Souls

Soul Devouring Presence. A creature that starts its turn within 30 ft of Blackrazor must succeed on a DC 18 WIS saving throw or become frightened until the end of its next turn. A creature frightened by this ability has disadvantage on saving throws against blackrazor's abilities. After saving a creature is immune to this feature for 24 hours.

Sentient Weapon. Blackrazor is fully sentient and possesses an INT, WIS, and CHA of its own. It can communicate telepathically with any creature within 120 ft. Blackrazor cannot be surprised

Regeneration. Blackrazor regains 8 hit points at the start of its turn for every soulmarked creature as long as it has at least 1 point

Shattered From. When Blackrazor hits zero hit points or if it takes more than 100 damage in one attack it can choose to shift to its shattered form.

Legendary Resistance(3/Day). If Blackrazor fails a saving throw, it can choose to succeed instead

Multiattack. Blackrazor makes a combination of 3 soulrend or soulpierce attacks.

Soulrend. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 14 (2d8 + 7) slashing damage and 2d8 necrotic damage(3d8 for any creature that is soulmarked)

Soulpierce. Ranged Weapon Attack: +10 to hit, range 30 ft., 1 target. Hit: 14 (2d6 + 7) necrotic damage and the target becomes soulmarked.

Void Slash (Recharge5-6).  Blackrazor releases a wave of screeching star-studded darkness in a 45 ft cone. Every creature in the area must make a DC 18 DEX saving throw, taking 5d8 cold damage and 5d8 necrotic damage on a failed save, or half as much on a success. Any creature that is soulmarked takes an additional 2d8 necrotic damage(OR the player cannot regain hit points until the end of their next turn, you can explain this thematically if anyone tries to heal a target) and the mark is consumed.

Mastery of Souls. Blackrazor can force a creature that has been soulmarked within 60 ft to make a DC 18 STR saving throw. On failure the creature is pulled 20 ft directly towards Blackrazor. (on success there is no effect and the soulmark is consumed?)

Soul Surge(2/Day). Blackrazor consumes one soul it has previously devoured, gaining the following benefits until the start of its next turn.

• It has advantage on attack rolls against soulmarked targets
• Its attacks deal an additional 1d8 necrotic damage against soulmarked targets
• All creatures hit within an attack roll becomes soulmarked
• It regains 1d6 hit points whenever it causes necrotic damage against a creature

Soul Needles. When a creature within 30 ft hits Blackrazor with an attack roll, Blackrazor can force the creature to make a DC 18 CHA save. On a failed save the creature feels inky black needles forcing themselves outwards from their soul. This causes 1d8 necrotic damage and the creature has disadvantage on attack rolls until the end of their next turn.

Shared Suffering. When Blackrazor hits a soulmarked creature with soulrend, it can soulmark another creature within 45 feet and force it to make a DC 18 CON save. On failure the creatures are linked and split the damage caused by soulrend until the start of Blackrazors next turn.

Enter the description for how legendary actions work for your monster here.

Space Step. Blackrazor can move up to half its movement and make one soulrend attack.

Soulmark. Blackrazor chooses one creature within 60 ft to soulmark.

Aspect of the Void(Costs 2 Actions). Blackrazor opens a rift next to a creature within 90ft. The creature must succeed on a DC 18 CHA saving throw, on a failure the creature is sucked in, taking 2d8 necrotic and 2d8 psychic damage reeling through the unfathomable cosmos. The rift closes and the creature returns to a point within range of the Blackrazors choice at the end of the creatures next turn. On a success the creature takes 2d8 necrotic damage and does not get moved.